1. Field of the Invention
The present invention relates to a sheet stacking device capable of staking a great many sheets.
2. Related Background Art
Conventionally, a sheet stacking device connected to, or accompanying an image forming apparatus such as a copying machine or the like has needed a spacing for stacking sheets between a sheet ejecting port to the sheet stacking device and a stacking tray, which is set equal to or larger than a thickness of a bundle of sheets to be stacked.
In the case of the sheet stacking device capable of stacking a great many sheets, a large spacing needs to be formed from the sheet ejecting port to the stacking tray in order to secure a stacking capacity.
However, if there is a long distance from the sheet ejecting port to an actual sheet stacking position, the sheets fall by a long distance to the tray after ejection, resulting in the difficulty of maintaining high stacking alignment.
Thus, the stacking tray of such a sheet stacking device includes a mechanism moved up and down according to a stacking quantity.
Specifically, the stacking tray is located in an upper position when a stacking quantity is small, and a distance from the sheet ejecting port is set short. Then, by lowering the stacking tray as a sheet staking quantity is increased, control is performed such that a distance from the sheet ejecting port to a position for stacking sheets on the stacking tray can be substantially constant.
However, since the weight of the stacking tray is heavy while a great many sheets are stacked thereon, a great force needs to be provided for lifting/lowering the tray.
Accordingly, it is necessary to increase an output by using a large motor or torque by reducing an elevating speed.
If the large motor is used, the stacking tray can be lifted/lowered at a relatively high speed even when the number of sheets to be stacked is large. Compared with the case of using a small motor, however, motor costs, installation space, power consumption, and so on, are larger. Thus, it is difficult to provide an inexpensive and compact device.
If an elevating speed is reduced by using the small motor, moving time becomes very long, for example when the stacking tray in a lowest position is moved to a highest position, and the sheets cannot be fed to the stacking device during such a period. Consequently, work efficiency falls.